Pramipexole, (S)-2-amino-6-n-propylamino-4,5,6,7-tetrahydro-benzothiazole, having formula (A),

is a dopaminergic agonist, known from U.S. Pat. No. 4,843,086, used in the treatment of Parkinson's disease in the form of the dihydrochloride monohydrate.
WO 2005/092871 discloses the synthesis of pramipexole and the salts thereof, starting from a compound of formula (I), as a individual (S) enantiomer, or a salt thereof,

wherein R is a protected amino group; and the asterisk * denotes the stereogenic carbon atom. A compound of formula (I), as the individual (S) enantiomer, can be obtained by hydrolysis of a mixture of (R,S) enantiomers of an ester of formula (II), or a salt thereof,

wherein R1 is straight or branched C1-C6 alkyl, optionally substituted with phenyl; and the asterisk * and R are as defined above, followed by resolution of the mixture of (R,S) enantiomers of the acid of formula (I), resulting in the individual (S) enantiomer. Alternatively, an acid of formula (I), as the individual (S) enantiomer, can be obtained enzymatically from a mixture of (R,S) enantiomers of an ester of formula (II), or a salt thereof.
The higher the enantiomeric purity of the starting compound of formula (I), the higher will be the enantiomeric purity of the final product pramipexole, obtained in WO 2005/092871. Therefore, an acid of formula (I), or a salt thereof, as the individual (S) enantiomer in said process typically has an enantiomeric purity of at least approx. 96%, preferably of at least approx. 99%. The preparation of a compound of formula (I), or a salt thereof, with such a high purity degree requires a number of steps, such as hydrolysis, recovery of the racemic intermediate (I), salification with an optically active base, recovery of the salt optically active, recrystallization and deprotection of the optically active acid. From the industrial point of view, these operations involve longer production times and higher costs. To date, the enzymatic preparation is also problematic and cannot be applied industrially, as the most suited enzyme as well as the optimum reactions conditions for said substrate are difficult to ascertain.
There is therefore the need for an improved process for the conversion of an ester of formula (II), as a mixture of (R,S) enatiomers, to the individual (S) enantiomer of formula (I).